638 JAMES L. KELLOGG 
determines whether food shall be taken, or all material rejected 
and sent outward. 
The operation of palp tracts for carrying food to the mouth 
has been described. Besides these, there are two other tracts, 
the function of which is to remove objectionable material from 
the palp surface. The most conspicuous of these is the ventral 
margin (vm), the entire surface of which is covered with cilia 
which lash posteriorly and carry material out to the narrow, free 
end of the organ, where it is cast off into the mantle chamber, 
and disposed of as previously described. Streams on this ventral 
margin are very rapid and powerful in all lamellibranchs. In 
one case among those examined (Chama, figs. 39, 40, 41) 
the direction of the stream is the reverse of its usual course, 
though its function is the same. A set of outgoing tracts 1s also 
found deep in all grooves between the palp folds of Schizotherus; 
and on them material is, at times, conducted from the palp sur- 
face to the ventral margin, of which these tracts are tributary 
streams. Here, then, on the palp face, are four distinct cilia 
systems. One is a general ciliation of the exposed faces of all 
folds, directing material anteriorly toward the mouth. A second 
consists of narrow, independent streams, one in a slight depres- 
sion on the. middle of each fold, all crossing. the first system, 
and directed dorsalward and forward. A third is a single broad 
band on the ventral margin, with the stream directed posteriorly. 
The fourth system is comprised of rather broad and powerful 
streams, joining the ventral margin, and ordinarily lying hidden 
in the deep grooves between the folds. They conduct material 
ventralward. When a pinch of sand is thrown on an exposed 
palp, and all these systems are observed in operation at once, 
the scene of confusion may be imagined. 
The adaptation in the case of each system, however, is clear. 
They operate under different conditions. It is my belief, after 
a good many years of observation, that lamellibranchs are able 
to feed only when the surrounding water is relatively free from 
solid particles—just how free, in a given case, I am not able to 
say, and the difficulties in determining the matter are great, if 
not insurmountable. What actually occurs on the palps, under 
